The docs referenced at http://tidy.sourceforge.net/docs/quickref.html above state that the configuration option 'sort-attributes' is an enumeration of 'none' and 'alpha', thereby specifying that strings of either form are the acceptable values. This may not be the case, however - on my system, the option was not honored until I set it to true. This may also be the case with other options, so experiment a bit. The output of tidy::getConfig() may be useful in this regard.